Method for coating the inner surface of metal pipes

ABSTRACT

A method for coating the inner surface of a metal pipe having an inner diameter of about 1 to 15 cm, which comprises feeding negatively charged dry particles of a coating composition into the interior of said metal pipe electrically grounded from one end of the pipe, sucking air at the same time from the other end of the metal pipe so as to form an air stream carrying the particles and flowing in the interior of the pipe at a flow rate of about 2 to 15 m/sec and to allow the particles to deposit electrostatically on the inner surface of the pipe, and fusing the deposited particles at a reduced pressure of up to about 100 mm Hg to form a continuous film on the inner surface of the pipe.

United States Paten [1 91 Inamura et a1. v

1 1 METHOD FOR COATING THE INNER I SURFACE OF METAL PIPES [75] Inventors: Keizolnamura; Tadayoshi Tatsuno;

Toshio Okoshi,.al1 of Hiratsuka, Japan [73] A'ssignee: Kansai Paint Company Limited,

Amagasaki-shi, Hyogo-l en, Japan 22 Filed: Mar. 20, 1973 21 Appl.No.:342,939

[30] Foreign Application Priority Data Mar. 25, 1972 Japan 47-29911 [56] References Cited UNITED STATES. PATENTS 2,280,240 4/1942 Kathe 117/119 2,869,511 11/1959 Dickey et al 2,880,109 3/1959 Current et a1. 117/119 2,974,059 3/1961 Gemmer 117/D1G. 6

[ Nov. 26, 1974 2,974,060 3/1961 Dettling ..l 117/D1G. 6 3,019,126 1/1962 Bartholomew.. 117/17 3,028,251 4/1962 Nagel 117/D1G. 6 3,074,808 11/1963 Harrison 117/21 3,186,860 '6/1965 Jonesnn; 117/21 3,199,491 8/1965 Biider et a1 1l7/D1G. 6

3,207,618 9/1965 DeHart r 117/21 3,394,450 7/1968 Gill et a1. 117/17 3,432,326 3/1969 Lemelson 117/21 3,439,649 4/1969 PrObSt 117/17 3,468,691 9/1969 Watkins 117/17 Primary ExaminerMichael Sofocleous Attorney, Agent, or Firm-Larson, Taylor & Hinds [57] ABSTRACT A method for coating the inner surface of a metal pipe having an inner diameter of about 1 to 15 cm, which comprises feeding negatively charged dry particles of a coating compositioninto the interior of said metal pipe electrically grounded from one end of the pipe, sucking air at the same time from the other end of the 'metal pipe so as to form an air stream carrying the particles-and flowing in the interior of the pipe at a flow rate of about 2 to 15 m/sec and to allow the particles to deposit electrostatically on the inner surface of the pipe, and fusing the deposited particles at a reduced pressure of up to about 100 mm Hg to form a continuous film on the inner surface of the pipe.

4 Claims, Drawing Figures PAIENTL mzslsm SHEET 10F 2 METHOD FOR COATING THE INNER SURFACE OF METAL PIPES This invention relates to a method for coating the tion Ser. No. 140,215 filed May 4', 1971 and Ser. No.

403,394, filed Oct. 4, 1973.

To protect a metal pipe from corrosion due to water;

steam, chemicals, etc. flowing in the pipe, it is-required to coat the inner surface of the pipe with a continuous resin film, and dry powder of coating compositions has been used for this purpose. In one of the known methods dry powder of a thermoplastic resin is fluidized in an air stream, in which a metal pipe preheated to a temperature higher than the melting point of the resin is immersed to form a resin film on the inner and outer surfaces of the pipe. According to this method, however, it is difficult to form a uniform pinhole-free coating on the inner surface of a pipe having an inner diameter smaller than about 15 cm, since resin particles can not be fluidized evenly in the interior of the pipe of such a small inner diameter. Further when a long pipe is treated in the above manner, it .has to be dipped in a fluidized bed of resin particles on a large-scale apparatus, and it is impossible to fluidize the resin particles uniformly on the interior surface of the pipe in the lengthwise direction thereof, with the result that an aneven film having numerous pinholes is formed. Since not only the inner'surface but also the outer surface of the pipe are coated simultaneously, this method has another disadvantage that it is practically impossible to provide the resin coating only on the interior surface of the pipe.

it is also known in the art to form a resin film on the inner surface of a pipe by electrostatic coating, using clear or pigmented dry powder of resins. This method employs an electrostatic powder spraying gun, socalled Pole gun," which is provided with a slidably extendable barrel and the coating is conducted by inserting the barrel of the gun into the interior ofa metal pipe electrically, grounded, spraying charged particles through the barrel to electrostatically deposit them on the inner surface of the pipe, and heating the pipe to produce a continuous coating film. However, this method is not suitable for coating the interior of a metal pipe ofa small inner diameter, because spark discharge frequently occurs between the high voltage electrode at the head of the gun barrel and the inner wall of the metal pipe, making it difficult to ensure uniform deposition of the charged particles. In fact, when a metal pipe having an inner diameter smaller than about 15 cm, particularly below about cm, is coated by this method, the resultant coating film formed on the inner surface of the pipe is uneven in thickness and has numerous pinholes throughout the film.

One object of the invention is accordingly to provide a method for coating the interior of'a metal pipe, which is capable of forming a uniform and continuous coating having excellent surface-smoothness free from pinholes on the inner surface of the pipe having such a small inner diameter as about 1 to cm.

Another object of the invention is to provide a method for coating the inner surface of a metal pipe in a continuous manner regardless of the length of the inner diameter as about 1 to 15 cm comprises feeding negatively charged dry particles of a coating composition into the interior of said metal pipe electrically grounded from one end of the pipe, sucking air at the same time from the other end of the metal pipe so as to form an air stream carrying the particles and flowing in the interior of the pipe at a rate of about 2 to 15 m/sec and to allow the particles to deposit electrostatically on the inner surface of the pipe, and fusing the deposited particles at a reduced pressure of up to 100 mm Hg to form a continuous film on the inner surface of the pipe.

According to the present invention, uniform and continuous fllm coating free from pinhole can be produced on the inner surface ofa metal pipe having such a small inner diameter that it has been difficult or impossible to form such a uniform pinhole-free film on the inner surface thereof by the conventional methods. In fact, the inner surface of a metal pipe having an inner diameter smallerthan about 15 cm, particularly-about l to 10 cm, can be effectively coated by the method of the invention. Moreover, the method of this invention makes it possible to form a continuous coating film having excellent surface-smoothness on the inner surface of a pipe not greater than about 15 cm in its inner diameter. Thus, the metal pipe which can be coated by the invention includes those having an inner diameter of about l to 15 cm and made of steel, stainless-steel,-aluminum, copper and like metals.

Various dry coating compositions heretofore used for electrostatic coatingmay be employed in the invention.

For example, polyvinyl chloride, polyethylene, polypropylene, polyamide, polyester, chlorinated polyether, epoxy resin, phenol resin, polyvinyl fluoride and like thermoplastic or thermosetting resins may be used as a coating composition in the invention in the form of dry powder, to which may be added, if necessary, plasticizers, stabilizers, coloring agents and like additives. Preferable particle size of the cocating compositions is in the range of about several microns to 500 a.

As the machine for giving electrostatic charge to the particles of the coating composition may be used various dry electrostatic coating machines available under the Trade Mark, such as REP Gun" sold by Ransberg Japan Ltd., Stajet and-*Stafluid by Societe Anonyme de Machines Electrostatiques, France, etc.

According to the process of the present invention, a metal pipe to be coated is electrically grounded. The metal pipe is preferably positioned horizontally, though it may be positioned vertically or in any direction desired. From one end of the pipe negatively charged dry powder of the coating composition is fed continuously into the interior of the pipe by a suitable dry electrostatic coating machine. At the same time air is sucked from the other end of the pipe by a suitable sucking device, such as suction pump, suction fan, etc.,- whereby an air stream is produced in the interior of the pipe. The dry powder fed from one end, while being carried by such air stream, is deposited electrostatically on the inner surface of the grounded pipe, resulting in uniform deposition of the powder on the entire surface of the pipe. To ensure uniform deposition, it is essential to control the flow rate of the air stream flowing in the interior of the pipe in the range of about 2 to 15 m/sec. lfthe flow rate is lower than about 2 m/sec, the powder is mainly deposited on a portion near the inlet of the pipe without uniform deposition being effected over the entire inner surface of the pipe, and at a higher flow rate of about 15 m/sec almost all powder particles are carried away with air and effective deposition can no longer be achieved. Particularlypreferable flow rate is in the range of @to 13 mlsec. a '7 To assure the deposition of the powder more uniformly it is preferable to rotate the metal pipe at least one r.p.m. during the electrostatic coating. The preferable rotation rate may be about to 50 r.p.m. though it may be increased to such a high rate as about 200 r.p.m.

The powder thus deposited on the inner surface of the pipe is then heated to fuse into a continuous coating film at a temperature lower than the decomposition temperature of the composition but higher than the melting point thereof. According to this invention, it is essential to fuse the composition especially at a reduced pressure of up to 100 mm Hg, preferably to 70 mm Hg, in terms of absolute pressure. The fusing conducted at the reduced pressure of up to I00 mm Hg achieves an outstanding effect of imparting remarkably improved surface-smoothness to the continuous coating film obtained. The continuous smooth coating formed on the inner surface of pipe reduces the resistance to fluids when the pipe is used for conveying water and other fluids, assuring a great advantage in the transportation of fluids. As the absolute pressure increases over 100 mm Hg, the surface-smoothness tends to reduce.

The interior pressure of the hollow metal pipe must be at a level not exceeding I00 mm Hg when the powder deposited on its inner surface is substantially fused. Accordingly, the interior pressure may be reduced to the above-mentioned level before or when the deposited powder reaches a temperature at which it starts to fuse. If the pressure is reduced after the deposited powder has already started fusing, a smooth-surfaced continuous film will not be formed on the inner surface of the pipe. The reduced pressure to be applied is up to I00 mm Hg, preferably in the range of about 10 to 70 mm Hg. The pressure reducing means is not limited in any way, but various means such as vacuum pump can be used effectively.

The thickness ofthe film thus obtained may vary over a wide range in accordance with the kinds of the coating compositions used and the time for coating, but usually it is in the range of about 100 to 700 u.

To make the coating process more efficient the metal pipe to be coated may be preheated at a temperature I of from a softening point of the coating composition to bellow a metling point thereof. By this pre-heating a thicker film can be obtained in a shorter period of time.

Throughout the specification and claims softening point" shows a value measured in accordance with ASTM D 1525-58 T, and melting point shows a value determined in accordance with ASTM D 1238-57 T, using a load of 2,160 g at a flow rate of lOi 1 g/lO min.

For better understanding of the invention, examples are given below in which the apparatus shown in the attached drawings is used.

FIG. 1 shows a side view partically in section of one preferred apparatus for carrying out the method of the invention;

FIG. 2 is a front view of rotating means shown in FIG.

Referring now to the drawings, designated at 1 is a couple of rotating means for a metal pipe to be coated, each of which comprises a driving roll 2, idle roll 3, set roll 4'and supporting means 5 for these rolls. The driving roll 2 and idle roll 3 are rotatably supported on shafts 6 and 7 on the supporting means 5, and the driving roll 2 is driven by bevel gears 8 and 9 which are driven through a reduction gear (not shown) by a motor (not shown), these reduction gear and motor being disposed in a case 10. The set roll 4 is rotatably supported on an arm 11 fixed to supporting means 5 with a screw 12.

A metal pipe A, the inner surface of which is to be coated, is electrically grounded and mounted horizontally on the couple of rotating means 1 and held in posi tion by the set roll 4 so as to be rotated by means of the driving roll 2. The metal pipe A is airtightly connected to a baffle 14 at the front end and to a rubber pipe 15 at the back end by means of socket and spigot joints 16 and union joints 17 respectively. Each union joint is supported by a frame 18. Designated at 13 is a heating furnace for the pipe A. The rubber pipe 15 is connected to a powder recovery hopper l9, and is further connected to air-sucking means (not shown) with a powder recovery box 20 disposed therebetween. The box is provided with a bag filter or screen 21 to prevent escape of the powder. Designated at 22 is a barrel head of a dry electrostatic coating machine (not shown).

The pressure reducing means, although not shown, may usually be a vacuum pump. For instance, the electrostatically coated hollow pipe is placed in an oven, and one end of the pipe is tightly closed with a heatresistant rubber cork, with the other end connected to the suction opening of a vacuum pump.

EXAMPLEv l The inner surface of a steel pipe, 5.5 m in length and 35 mm in inner diameter, was coated in the following manner on the apparatus shown in attached drawings.

The pipe mounted on the rotating means 1 and electrically grounded was rotated at 5 r.p.m. and heated at a temperature of about C. A powdery coating composition comprising epoxy resin, hardener and pigment and having the particle size of 20 to p. was charged negatively to a voltage of -90 KV and blown through the electrostatic gun head 22, REP Gun" (Trade Mark), to the baffle 14 at the rate of 300 g/min. At the same time air was sucked by means of a suction pump, whereby air stream flowing through the interior of the pipe at a flow rate of 10 m/sec was produced. The dry powder blown was carried by the air stream and deposited electrostatically on the inner surface of the pipe. This procedure was continued for 2 minutes.

Subsequently, the hollow steel pipe A with the coating composition electrostatically deposited on its inner surface was tightly closed at its one end by silicon rubber cork and connected at the other end thereof to a suction opening ofa vacuum pump by way ofa manometer. While maintaining the interior of the steel pipe A at a reduced pressure of 30 mm Hg, the pipe A was heated in an oven to l C for about 10 minutes (i.e.,

the fusing temperature of the powdery epoxy resin coating composition) and then the interior pressure of the steel pipe A was returned to the atmospheric pressure, followed by further'heating to 180C. The heating.

at that temperature was continued for 30 minutes to cure the epoxy resin. As a result, a hollow steel pipe was obtained which was coated on its interior surface with a film of the epoxy resinhaving an almost uniform thickness of about 250 ,u. When the coated surface of the steel pipe was subject to discharge at a voltage of 1,500 V in contact with a pinhole tester (Trade Mark:

' Poroscope H 2e, product of HELMUT FISCHER G.m.b.H., West Germany), no spark was observed to EXAMPLE 5 Thepowder coating composition electrostatically detake place. Thus it was ascertained that the coating film EXAMPLE 2 I Electrostatic coating was conducted in the same manner as in Example -l, except that the flow rate of the air stream was 3 m/sec.

The resultant film was uniform free of pinholes and had a thickness of 210 a. The film was highly smoothsurfaced.

EXAMPLE 3 .the latter case almost no deposition of the dry powder was observed.

EXAMPLE 4 The inner surfaces of the steel pipes having different inner diameters were electrostatically coated in the same manner as in Example I, with the results shown in Table 1 below, in which the surface conditions of the resultant film were inspected in the same manner as in Example 1.

Table 1 No. lnner Thickness Surface condiu. of film ditions uf film of pipe (11.)

1 |2.7 I90 Uniform free Excellent in of pinhole surface smoothness 2 35.7 240 do. do. 1 l()5.3 330 do. do.

posited on pipes in the same manner as in Example 1 was heated and fused to form a continuous coating film by followin g the same procedure as in'Example 1 ex-- cept that the fusing of the deposited particles was conducted at varying reduced pressures. The results are given' in Table 2 below.

Table 2- N0. Reduced Thickness pressure I of film Surface-smoothness in fusing (l 4 10 200 Excellent 5 50 250 Excellent 6 250 Good 7 290 Fair 8 200 300 Poor 9 760 280 Poor EXAMPLE 6 The inner surface ofa steel pipe, 5.5 m in length and 40 mm in inner diameter was electrostatically coated in the same manner as in Example 1, except that the flow rate of the air stream was about 3 m/sec, pipe preheating temperature was 60C, and the coating composition was a powder coating composition of polyamide comprising 80 percent by weight of polyamide resin, 15 percent by weight of pigment and a small amount of plastisizer and having particle sizes of about 30 to 200 Then the hollow steel pipe with the polyamide composition electr'ostatically deposited onits interior surface was heated at 200C in an oven for 30 minutes to fuse the polyamide composition while maintaining the interior of the hollow pipe at an absolute pressure of 10 mm Hg.

Subsequently, the interior pressure of the pipe was returned to the atmospheric pressure and the pipe was cooled to room temperature, whereby a hollow steel pipe was obtained which was coated on its interior surface with a uniform film of the polyamide composition having an average thickness of about 200 t. The surface of the film was very smooth. When tested for pinholes in the same manner as in Example 1, the film was found to be free of any pinhole. Further when the coated pipe was tested for its resistance to corrosion in the same manner as-in Example 1, no rust was observed even after it was immersed in a 5 wt. percent sodium chloride solution for 1,000 hours.

EXAMPLE 7 The electrostatic coating was conducted in the same manner as in Example 6, except that the flow rate of the air stream was 13 m/sec.

The resultant film of 380 p. was formed on the entire inner surface of the pipe and was uniform free of pinhole.

' For comparison electrostatic coating the same as in Example 6 was carried out at the flow rate of 0 m/sec and 18 m/sec. In the formercase dry powder was deposited only on the front part of the pipe with almost no deposition on the back part, failing to produce uniform film, and in the latter case almost no deposition of the'dry powder was observed.

' EXAMPLE 8 The inner surfaces of the steel pipes having different inner diameters were electrostatically coated in the same manner as in Example 6, with the results shown in Table 3 below.

Table 3 No. lnner dia. Average of pipe thickness Surface conditions (mm) of film of film 16.1 310 Pinholefree and smooth surface I 1 52.9 340 do. I2 105.3 300 do.

EXAMPLE 9 Various metal pipes made of copper, stainless-steel and aluminum,,respectively 5.5 m in length and 35.7 mm in inner diameter, were electrostatically coated in the same manner as in Example 6, with the results shown in Table 4 below.

The inner surface ofa steel pipe, 5.5 m in length and 35.7 mm in inner diameter was electrostatically coated in the same manner as in Example 6 withvarious dry coating compositions of a particle size of about to 200 p. shown in Table 5 below, in which the results are'- also shown.

' Table 5 No. Coating Comp. Average Surface conditions thickness of film of film l6 Polyvinyl 310 Pinhole-free and chloride smooth surface 17 Polyethylene 360 do. [8 Polyester 310 do. 19 Phenol resin 32Q do. g 20 Chlorinated 340 do.

polyether 2] Fluorine resin 290. do.

What we claim is:

l. A method for coating the inner surface of a metal pipe having an inner diameter of l-l5 cm with a dry coating composition, which comprises the steps of:

a. rotating said metal pipe electrically grounded at a rate of at least 1 R.P.M.,

b. pre-heating said metal pipe at a temperature of from a softening point of said coating composition to a temperature lower than a melting point thereof,

c; feeding negatively charged dry particles of a coating composition into the interior of the metal pipe from one end of the pipe,

d. sucking air at the same time from the other end of the metal pipe so as to form an airstream carrying the particles and flowing in the interior of the pipe at a flow rate of 2-15 m/sec. and to allow the particles to deposit electrostatically on the inner surface of the pipe, and

e. fusing the deposited particles at a reduced pressure of up to about 100 mm Hg to form a continuous film on the inner surface of the pipe.

2. The method for coating the inner surface of a metal pipe according to claim 28, in which said flow rate of air stream is in the range of 3 to 13 m/sec.

3. The method for coating the inner surface of a metal pipe according to claim 1, in which said step of rotation of said metal pipe is at a rate of 5 to 50 rpm.

4. The method for coating the inner surface of a metal pipe according to claim 1, in which the reduced pressure is in the range of 10 to mm Hg. 

1. A METHOD FOR COATING THE INNER SURFACE OF A METAL PIPE HAVING AN INNER DIAMETER OF 1-15 CM WITH A DRY COATING COMPOSITION, WHICH COMPRISES THE STEP OF: A. ROTATING SAID METAL PIPE ELECTRICALLY GROUNDED AT A RATE OF AT LEAST 1 R.P.M., B. PRE-HEATING SAID METAL PIPE AT A TEMPERATURE OF FROM A SOFTENING POINT OF SAID COATING COMPOSITION TO A TEMPERATURE LOWER THAN A MELTING POINT THEREOF, C. FEEDING NEGATIVELY CHARGED DRY PARTICLES OF A COATING COMPOSITION INTO THE INTERIOR OF THE METAL PIPE FROM ONE END OF THE PIPE, D. SUCKING AIR AT THE SAME TIME FROM THE OTHER END OF THE
 2. The method for coating the inner surface of a metal pipe according to claim 28, in which said flow rate of air stream is in the range of 3 to 13 m/sec.
 3. The method for coating the inner surface of a metal pipe according to claim 1, in which said step of rotation of said metal pipe is at a rate of 5 to 50 r.p.m.
 4. The method for coating the inner surface of a metal pipe according to claim 1, in which the reduced pressure is in the range of 10 to 70 mm Hg. 